Mobile positioning technologies that utilize measurements of wireless communication signals are emerging as viable options in mobile communication systems today. Some mobile positioning systems have been developed for location based services (LBS) in code division multiple access (CDMA) based mobile communication systems but they still have problems related to mobile communication channels and systems. In CDMA or wideband code division multiple access (WCDMA) based mobile communication systems, hearability is one of the most critical and practical challenges to using its wireless communication signals for positioning.
The academy and industry have continually engaged in efforts to develop reliable positioning technologies for mobile phone users. The emergency 911 (E911) requirement, for emergency rescue service, enforced by the United States (US) Federal Communications Commission (FCC) dramatically increased the interest in mobile positioning technologies. In 1999, the FCC tightened the Phase II location accuracy requirement. This requirement demanded handset-based solutions provide a location accuracy of 50 meters in 67 percent of calls and 150 meters in 95 percent of calls. The requirement further required that network-based solutions provide a location accuracy of 100 meters in 67 percent of calls and 300 meters in 95 percent of calls for handset-based solutions. Handset-based solutions are for non-legacy phones and network-based solutions are for legacy (communications-only) phones. The executive body of the European Union (EU) is working on similar initiatives for their wireless emergency calls, E112. Mobile communication service providers are also required to support their customers with location based services (LBS).
Some positioning technologies have been accepted and deployed in second generation mobile communication systems such as the Global System for Mobile Communication (GSM) standardized by European Telecommunication Standard Institute (ETSI) and the Interim Standard 95 (IS-95) standardized by Telecommunication Industry Association (TIA) of United States. GSM uses Time Division Multiple Access (TDMA) technology where mobile terminals (known as mobile stations in IS-95—‘mobile station’ will be used instead of ‘mobile terminal’ hereafter) are allocated their own time slots and frequency channel assignments. IS-95 is based on Code Division Multiple Access (CDMA) technology where mobile stations simultaneously share time and frequency resources.
Unlike IS-95 mobile stations, GSM mobile stations have a better chance of receiving signals transmitted from neighboring base stations without having large interference from the service base station. Enhanced Observed Time Difference (E-OTD) positioning system, in GSM, utilizes Time Difference of Arrival (TDOA) measurements of received wireless signals that are transmitted from GSM base stations. E-OTD shows that positioning systems utilizing wireless communication signal measurements can provide sufficient positioning accuracy and reliability. Universal Mobile Telecommunication Systems (UMTS), the third generation mobile communication system standardized by third Generation Partnership Project (3GPP), utilizes Wideband CDMA (WCDMA) technology for its Common Air Interface (CAI). Therefore, IS-95 mobile stations and UMTS user equipments (UEs), same system elements as mobile stations in IS-95, experience difficulties in measuring TDOA of received wireless signals transmitted from neighboring base stations due to the interference from the service base station.
In general, the service base station is the closest base station to the mobile station so that the received signal strength (RSS) of the signal from the service base station is much larger than the RSS of signals from all other neighboring base stations. This is because IS-95 and UMTS are CDMA based mobile communication systems in which a mobile station receives signals transmitted from all of the surrounding base stations continuously. The presence of strong interference from the service base station is one of the most difficult problems to overcome for mobile positioning systems trying to obtain multiple measurements for received wireless signals transmitted from multiple base stations in IS-95 and UMTS. This problem is sometimes referred to as the hearability problem.
These and other issues have presented challenges to the implementation of the mobile station position, including those involving cellular communications and those used in similar applications.